1. Field of the Invention
The present invention relates to a television broadcast signal receiver for receiving digital television broadcast signals such as ATSC (Advanced Television Systems Committee).
2. Description of the Related Art
In the North American Continent such as the United States of America where ATSC digital television (hereafter referred to simply as “TV broadcast”) is broadcast, big cities spread across the plains. From the viewpoint of a television user or viewer to receive TV broadcast signals, which are broadcast in big cities or their suburbs, the TV broadcast signals or even one TV broadcast signal may come from various directions. Thus, it is necessary for the user to adjust the direction of an antenna in the direction of a broadcast tower transmitting a television broadcast signal of a television program which the user wishes to watch.
For this reason, multi-directional antennas such as a so-called smart antenna are put to practical use. On the other hand, general antennas for receiving terrestrial broadcast signals have a shape of the so-called Yagi antenna, which has a high directivity to be able to receive a weak radio wave or signal. At the same time, the Yagi antenna has a drawback that it can receive only a radio wave from one broadcast tower due to the high directivity, hence the Yagi antenna being a unidirectional antenna. Due to the high directivity, furthermore, even a slight variation in the receiving direction of the Yagi antenna may cause a significant variation in the signal intensity of a received TV broadcast signal.
Meanwhile, several methods are known for controlling the receiving direction of an antenna. For example, Japanese Laid-open Patent Publication Hei 05-267919 discloses a method for controlling to bring an antenna to a best receiving condition by using a sensor for detecting a rotation angle of the antenna. Further, Japanese Laid-open Patent Publication Hei 06-225230 discloses a method for controlling rotation of an automotive BS (Broadcast Satellite) receiving antenna by generating a direction signal and a drive signal responding to signal level determination. Still further, Japanese Laid-open Patent Publication 2003-344517 discloses a method for controlling an automotive BS receiving antenna to track a satellite by varying a rotational speed of the antenna as the signal intensity received by the antenna varies.
As described above, according to the ATSC digital television broadcast in the United States of America, the broadcast signals or waves may come from various directions, so that multi-directional antennas such as a smart antenna are needed to receive such TV broadcasts. The EIA (Electronic Industries Association)-909 standard provides a smart antenna having sixteen equiangular receiving directions in 360 degrees. A digital TV (television) broadcast signal receiver conforming to the EIA-909 standard is set to be able to adjust or switch an active signal receiving direction of the multi-directional antenna such as the smart antenna to every one of the sixteen receiving directions.
Some of such digital TV broadcast signal receivers have an omni-directional scanning function (automatic scanning function) to select the receiving direction of an antenna in which a digital TV broadcast signal of a channel selected by a user can be best received. This can be done by scanning the sixteen directions for each channel, and by setting the best receiving direction as a receiving direction for the channel, because usually the position of a broadcast antenna or a broadcast tower of a broadcast station does not change. However, in countries like the United States of America, broadcast stations may go into bankruptcy while it is relatively easy to open new broadcast stations. Accordingly, channels which could not previously be received may become receivable, while channels which could previously be received may become unreceivable. Furthermore, the receiving conditions of digital TV broadcast signals may change due to newly built high rise buildings. For this reason, it may be necessary to reset the best receiving direction for the channel selected by the user.
In such case, a multi-directional antenna which can switch the receiving direction in a short time can be connected to a digital TV broadcast signal receiver having the omni-directional scanning function. However, even with the multi-directional antenna, it may take a very long time to determine a best receiving direction of the antenna for receiving a TV broadcast signal of the channel selected by the user because a receiving direction control signal for switching the receiving direction may be output at very long intervals, thereby causing the user to get irritated. For solving this problem, a multi-directional antenna such as a smart antenna is designed such that at an initial setting, best receiving directions and maximum signal levels of received TV broadcast signals for all local channels in all directions are measured by taking time in advance, and are stored in a built-in memory of a TV broadcast signal receiver. For channel selection, the memory can be accessed to enable immediate adjustment of the antenna to point at a broadcast tower for a desired channel.
However, when e.g. an indoor antenna is used, it is quite probable that a user would change a place of installing the antenna, or physically change a receiving direction of the installed antenna by mistake, making it likely to generate a trouble that TV images suddenly become unreceivable. In such case, once the receiving direction of the antenna changes, data of the receiving directions for respective channels stored in the TV broadcast receiver become inaccurate and meaningless. In order to restore a normal receiving condition, it is required to perform an omni-directional scanning process and measure signal intensities or levels in all receiving directions of the antenna for all channels again from the beginning. This causes a problem that requires a very laborious work and an unnecessary consumption of time and energy.